"We know what Nexus means now. There can no longer be any doubt: a Nexus device is about openness first and foremost. That does not mean Google won't make compromises with the Nexus program. It simply means that Google will only make compromises when it comes to increasing openness. Why? Because Google benefits from open devices as much, or more than you do. Last year the technology sphere was busily discussing whether or not the Verizon Galaxy Nexus was a 'true' Nexus device. This year we have an answer: a Nexus controlled by a carrier is no Nexus. Rather than get in bed with Verizon, Sprint, or AT&T to produce an LTE version of the Nexus 4, we have HSPA+ only. Even the new Nexus 7 with mobile data is limited to this enhanced 3G standard." Interesting take on the whole thing - reeks a bit of finding a reason for a feature deficiency, but it does fit into the available facts.

I guess openness doesn't include GPU drivers and etc. which are required to run conventional Linux (i.e. non Android) on those devices.

It also doesn't include the source code for the Google apps. When these for-profit corporations start talking about openness, what they really mean is 'openness, as long as it's convenient for us'. Which is fine, as long as you don't fall for their bullshit rhetoric, like they're somehow doing you a favor.